Patent Documents 1 and 2 disclose a reclining device which includes: a guide bracket fixed to one of a seat cushion and a seat back; an internal gear fixed to the other; and lock plates disposed between the guide bracket and the internal gear and having external teeth engageable with internal teeth of the internal gear, and which keeps the seat back at an appropriate angle of inclination by making the internal gear and the lock plates engaged with each other and allows the seat back to turn forward and backward by releasing their engagement. A mounting ring is fixed to an outer peripheral surface of the guide bracket or the internal gear. The mounting ring is formed in a ring shape with a substantially L-shaped cross section having a bottom surface portion and a peripheral surface portion and is inserted from one of the guide bracket and the internal gear, its peripheral surface portion is fixed by welding or the like to the outer peripheral surface of the guide bracket or internal gear located on a far side in an insertion direction, and the internal gear or guide bracket located therebetween is rotatably supported between the guide bracket or internal gear and the bottom surface portion. This is suitable for a thickness reduction because locking is made possible by the engagement of the lock plates and the internal teeth of the internal gear which are housed inside.